Most noise from an internal combustion engine propagates from the engine's exhaust ports and has commonly been suppressed by means of mufflers, also known as silencers, located in an exhaust pipe connected to the ports. Such mufflers are entirely passive devices.
It has been proposed to use active noise control techniques to control the noise propagating along the exhaust pipe. In one such system, the exhaust pipe is caused to pass through a chamber, in which a loudspeaker is mounted. The loudspeaker is used to produce anti-noise in the chamber, for cancelling the exhaust noise. It has been found that in order for there to be sufficient acoustic coupling between the loudspeaker and the inside of the exhaust pipe, the portion of the exhaust pipe passing through the chamber should be perforated. However, a problem with this arrangement is that exhaust gases pass out through the perforations located towards the chamber inlet. This escape of exhaust gases results in a pressure differential along the length of the perforated portion of the pipe. As the pressure in the perforated portion of the pipe is lower towards the chamber outlet than towards the chamber inlet, exhaust gases in the chamber are drawn back into the exhaust pipe through the perforations located towards the chamber outlet. Thus, there is a constant flow of hot exhaust gases through the chamber during operation. These hot exhaust gases are deleterious to the loudspeaker mounted in the chamber.
In an alternative arrangement, an inflow pipe passes completely through the chamber and terminates within an outflow pipe. However, it has ken found that when such an arrangement is used in a virtual earth noise cancellation system, the positioning of the system microphone is extremely critical. Even with the microphone optimally placed, it is still not possible to achieve effective cancellation over a wide range of frequencies.
It is an object of the present invention to overcome the aforementioned disadvantages of the prior art and provide a muffler which enables satisfactory acoustic coupling to the interior of an exhaust pipe while providing a benign environment for a loudspeaker.